With ever increasing demands for international trade and commerce, it is becoming more difficult to monitor the importation or smuggling of dangerous materials into a country. One very dangerous item that may be illegally imported into a country is nuclear material, such as highly enriched uranium (HEU). While any undeclared nuclear material is of concern and needs to be detected, HEU is of particular concern. HEU is a primary nuclear weapons component and can be used in limited quantities to produce an effective weapon of mass destruction. HEU is very difficult to detect in any shielded configuration due to its very low radioactive decay emissions. For example, HEU has a half life of 3.5×1017 years with a neutron emission of 2×10−4 n/s/g and a 186-keV gamma-ray emission that is easily shielded. Therefore, countries have to monitor their borders for nuclear materials in order to prevent adverse parties from building and detonating nuclear-type weapons within their borders.
The problem of detecting the importation of nuclear materials is exacerbated by the high quantity of international trade in the world. For example, a port-of-entry typically has too many containers entering the country to individually inspect each container for nuclear materials and other contraband. The major problem with any manual container inspection is that the inspection process is extremely tedious and costly due to the large container sizes and voluminous quantities of goods involved.
Nondestructive detection techniques of nuclear materials are characterized as passive or active depending on whether they measure radiation from the spontaneous decay of the nuclear material or from the radiation induced by an external interrogating source. Passive techniques can provide some capability in detecting nuclear materials; however, these techniques are limited due to the wide variety of possible nuclear material shielding configurations and the physical positioning within these large cargo containers. As indicated previously, the detection of HEU is even more difficult for passive detection techniques due to its very low radiation emissions.
Low radiation emission levels from nuclear materials, whether emitted directly or as a result of attenuation from neutron/gamma shielding, require very sensitive detectors to enable detection. Even with the most sensitive detectors, the detection of nuclear material is limited by the detectors' ability to distinguish natural background signals from the radiation generated by the nuclear material. Hence, in most cases involving containerized cargo, the passive detection techniques are challenged in the detection of most shielded nuclear materials, and especially shielded HEU.
One active interrogation technique, using an external neutron source (isotope or neutron generator), can be used to detect nuclear materials via the neutron multiplication effect from the fissioning-events in nuclear materials. Unfortunately, discriminating between the external neutrons emitted by the interrogating source and the induced neutrons from the nuclear material is difficult within a large-volume environment, such as a cargo container. In addition, this technique is application limited because of the administrative restrictions on the required neutron source strength involved and the associated neutron energy, slowing-down considerations.
Therefore, a need exists to overcome the above-described problems.